It is called a piezochromic phenomenon that optical properties of a material change reversibly according to a change of pressure. Samarium monosulfide (SmS) is a piezochromic material characterized by the optical properties being able to change upon pressure via a semiconductor-to-metal isostructural phase transition at 0.65 MPa and exhibits piezochromic characteristics. When a thin film thereof is used, for example, as a window coating material, it becomes possible to automatically control an energy of incident sunlight to be passed through the window according to a change of circumstantial temperature and stress thereof (including thermal stress introduced from a change of the temperature (proposed by the present inventors)). The pressure for phase transition of SmS is comparatively low among the known piezochromic materials and the phase transition is accompanied by a large change in optical properties from the visible wavelength to the infrared. The excellent optical switching property makes the material attractive for applications such as nonvolatile memory and possibly other kinds of smart devices with both sensing and actuating functions. An optical switching element, an information recording element, a non-linear optical device and the like which are utilizing a change of optical characteristics of the material have been reported [e.g., C. B. Greenberg: Thin Solid Films 251 (1994) 81.].
With respect to methods of the synthesis of a samarium monosulfide thin film, there have been examples of synthesis by CVD, a method of vacuum deposition and explosive evaporation and the like [e.g., 1) M. R. Jacobson, P. D. Hillman, A. L. Phillips, U. J. Gibson: SPIE Proc., vol. 428 (1983) 57., 2) C. F. Hickey and U. J. Gibson: J. Appl. Phys. 62 (1987) 3912., 3) Dieter W. Pohl and F. Holtzberg: Appl. Opt. 14 (1975) 1060., 4)V. V. Kaminskii, N. M. Volodin, T. B. Zhukova, M. V. Romanova, and G. A. Sosova: Sov. Phys. Solid State 33 (1991) 108.]. With respect to a sputtering process, an example of the synthesis of a samarium monosulfide thin film wherein H.sub.2 S is employed as a reactive gas has been reported; however, the process of the synthesis employs H.sub.2 S, a toxic gas which is harmful to circumstances and health, and besides the synthesized thin film is powdery, does not adhere to a substrate and is chemically unstable film [M. R. Jacobson, P. D. Hillman, A. L. Phillips, U. J. Gibson: SPIE Proc., vol. 428 (1983) 57.]. Since stoichiometric SmS target could not be commercially obtained, there existed no example of synthesis of the film by employing the stoichiometric SmS target. Only the semiconductor phase were formed in most cases and it required mechanical polishing to transfer it into metal phase. There was one sturdy on direct formation of metal phase mixed with Sm (up to 40% wt %) by simultaneously sublimating Sm and S in vacuum.
As described above, several processes for producing a samarium monosulfide thin film including a sputtering process have been reported. However, it is apparent that the above-mentioned conventional techniques of production of the thin film have several disadvantages that a toxic gas such as hydrogen sulfide is used as a reactive gas, or that the thin film synthesized may contain a large amount of impurities, or that the structure control of the thin film is difficult or the like. Most of the thin films synthesized comprise a semiconductor phase, and it is necessary to pressurize the thin films by mechanical polishing them in order to cause a phase transition to a metal phase thereof [T. L. Bshalava, T. B. Zhukova, I. A. Smirnov, S. G. Shulman, and N. A. Yakovleva, Sov. Phys. Solid State 16 (1975) 2428.].
The present inventors have engaged in assiduous studies, under these circumstances, with a view to preparing a samarium monosulfide piezochromic thin film which can be comprising not only a semiconductor phase but also a metal phase stable at atmospheric pressure by using a new sputtering deposition process which is suited for industrial production characterized in that the film is synthesized without employing any toxic gas, at a high film formation rate, with few impurities, and as a result have accomplished the present invention.